Scavenge Exploit
You can duplicate resources if you plan ahead. As you search for supplies, occasionally you will find Heath, Ammo, Fuel, or Food in packs. Once you do, press DOWN on the d-pad and you can choose "Call for scavengers" to call someone to search the area. When you receive confirmation that someone is on the way, grab the resource pack. When the scavenger arrives, that same will be snagged a second time, in spite of the fact that you just took it. In this manner, you can obtain twice the supplies you should have received.

Stuck Car Horn
As you are driving a vehicle, you can press and hold the left thumbstick to activate your vehicle's horn. Continue holding that button and press the BACK button to bring up the map. The car horn should still be sounding at that point. Now let go of the thumbstick and press BACK again. The horn should continue to blare, and will do so until you press in on the thumbstick again. This could prove useful in distraction missions.